Chlorine is produced almost entirely by electrolytic methods, primarily from aqueous solutions of alkali metal chlorides. In the electrolysis of brines, chlorine is produced at the anode and hydrogen, together with an alkali metal hydroxide, such as sodium or potassium hydroxide, at the cathode. As the anode and cathode products must be kept separate, many cell designs have been developed. These designs have generally utilized either a diaphragm, or a mercury intermediate electrode to separate the anolyte and catholyte products.
In the diaphragm process, brine is fed continuously into the electrolytic cell and flows from the anode compartment through an asbestos diaphragm into the catholyte compartment which contains, for example, an iron cathode. To minimize back-diffusion and migration, the flow rate is always such that only part of the salt is converted. The hydrogen ions are discharged from the solution at the cathode, forming hydrogen gas and leaving hydroxyl ions. This catholyte solution, which contains sodium hydroxide and unchanged sodium chloride, is evaporated to obtain the sodium hydroxide. In the course of the evaporation the sodium chloride precipitates, is separated, redissolved, and sent back into the electrolytic cell. The function of the diaphragm is to maintain the level of concentration of alkali, to minimize the diffusional migration of hydroxyl ions into the anolyte and to maintain separation of hydrogen and chlorine. The diaphragm should also have minimal electrical resistance.
In the mercury electrode process, the cation, after discharge, forms an alloy or analgam with mercury. The amalgam flows or is pumped to a separate chamber in which it is allowed to undergo galvanic reaction, most often with water, to form hydrogen and a comparatively strong sodium hydroxide solution containing almost no sodium chloride.
The diaphragm process is inherently cheaper than the mercury process, but as the former process does not provide chloride-free alkali, additional processing steps are necessary to purify and concentrate the alkali.
Substitution of an ion-exchange membrane material for the diaphragm has been proposed. Numerous membrane materials have been suggested. For example, membranes are described in U.S. Pat. Nos. 2,636,851; 2,967,807; 3,017,338; and British Pat. Nos. 1,184,321 and 1,199,952.
Such membranes are substantially impervious to hydraulic flow. During operation, brine is introduced into the anolyte compartment wherein chlorine is liberated. Then, in the case of a cation permselective membrane, sodium ions are transported across the membrane into the catholyte compartment. The concentration of the relatively pure caustic produced in the catholyte compartment is determined by the amount of water added to this compartment from an external source, and by migration of water, in the cell, i.e. osmosis and/or electro-osmosis. While operation of a membrane cell has many theoretical advantages, its commercial application to the production of chlorine and caustic has been hindered owing to the often erratic operating characteristics of the cells. A number of disadvantages have been present when using these membranes, including a relatively high electrical resistance, poor permselectivity and oxidative degeneration, as well as relatively high cost.
As an alternative to asbestos diaphragms and impermeable ion exchange membranes, the industry has sought a suitable porous or microporous plastic diaphragm material. The principle involved consists of producing a thin electrically conductive chemically resistant plastic sheet material having the desired degree of porosity.
Numerous references may be found relating to such membrane materials. Mention may be made more particularly to the following patents which employ techniques of compression preforming followed by fritting, or sintering, or techniques of coagulation of the mixture or the deposition of this mixture on a support.
French Pat. No. 1,491,033, of Aug. 31, 1966, relates to a process for manufacturing porous diaphragms which consists of mixing a solid additive in particulate form into an aqueous dispersion of polytetrafluoroethylene in the presence of particulate inorganic fillers, coagulating the dispersion, placing the resultant coagulum in sheet form, and removing the solid particulate additive from the sheet. The removable particulate additive generally consists of starch or calcium carbonate, and is removable by immersion of the resultant sheet in hydrochloric acid. Alternatively, the additive may also be a plastic polymer which is soluble in an organic solvent, or depolymerizable, or evaporatable upon heating of the sheet. The particulate inorganic fillers which are suitable include barium sulfate, titanium dioxide, and asbestos.
U.S. Pat. No. 3,890,417, issued June 17, 1975, teaches a method of manufacturing a porous diaphragm comprising preparing an aqueous slurry or dispersion comprising polytetrafluoroethylene and a solid particulate additive, thickening of the aqueous slurry or dispersion to effect agglomeration of the solid particles therein, forming a dough-like material containing sufficient water to serve as a lubricant in a subsequent sheet forming operations, forming a sheet of desired thickness, and removing the solid particulate additive from the sheet. The solid particulate additive can be any which is substantially insoluble in water, but which is removable by a suitable chemical or physical means. Examples indicated are starch, and calcium carbonate. U.S. Pat. No. 3,281,511, issued Oct. 25, 1966, discloses preparing microporous polytetrafluoroethylene resin sheets by mixing fine polytetrafluoroethylene powder with a carrier and a readily removable filler material, rolling the thus made dough with intermediate reorientation, so the particles are biaxially oriented. The solvent is then evaporated and the polytetrafluoroethylene is sintered at above its melting temperature, followed by removal of the filler by an appropriate solvent. The carrier material is a readily vaporizable material such as a naphtha or petroleum distillate, such as Stoddard solvent, which is a standard petroleum distillate having a flash point not lower than 100.degree. F., comprised largely of saturated hydrocarbons.
U.S. Pat. No. 3,556,161, issued Jan. 19, 1971, relates to polytetrafluoroethylene sheet materials formed by the "slip-forming" process, comprising mixing polytetrafluoroethylene powder with a liquid such as kerosene, and then sequentially working the resultant composition by the application of concurrent compressive stress and shear stress, the sequence of operations being directed so that the shear stress components are distributed substantially biaxially, resulting in planar orientation in the resulting article. As is the case with the material found by the process of U.S. Pat. No. 3,281,511, the sheet material formed is biaxially oriented, and of high tensile strength.
These and other well known techniques, in the case of membranes rich in polytetrafluoroethylene, have not been capable of producing membranes of satisfactory mechanical properties, i.e. proper porosity, and good wettability.
It is an object of the present invention to provide a novel and improved method of producing microporous separators suitable for electrolytic cells. It is also an object of the invention to provide an improved separator for use in chlor-alkali cells and fuel cells which has a low electrical resistance, and behaves like a porous medium, having a porosity greater than about 70 percent, permitting both the passage of the current and the uniform flow of electrolyte from one compartment of a cell to another.